Identification of a novel mutation in the PRCD gene causing autosomal recessive retinitis pigmentosa in a Turkish family

Purpose Progressive rod-cone degeneration (PRCD) is a canine form of autosomal recessive photoreceptor degeneration and serves as an animal model for human retinitis pigmentosa (RP). To date, only two RP-causing mutations of the PRCD gene have been reported in humans. We found a novel mutation in PRCD (c.52C>T, p.R18X) in three siblings affected by RP and present detailed morphologic and functional parameters. Methods A complete ophthalmological examination was performed including psychophysical tests (best-corrected visual acuity, Lanthony Panel D-15 color vision test, and visual field) and electrophysiology (ganzfeld and multifocal electroretinogram). Additionally, color and infrared fundus photography, autofluorescence, and spectral domain optical coherence tomography recordings were performed. Genomic DNA of the three affected individuals was analyzed with high-throughput sequencing for all RP-related genes in a diagnostic set-up. Results We identified a novel homozygous mutation in PRCD (c.52C>T, p.R18X) with diagnostic high-throughput panel sequencing. All three patients showed an advanced stage of retinitis pigmentosa with reduced visual acuity (mean: 20/80), small residual visual fields (mean for target III4e: 1134.35 deg2), and non-detectable electrophysiological responses. Myopia, posterior subcapsular cataract, bone spicule-like pigmentation, and attenuated arterioles were typical findings. Interestingly, bull’s eye maculopathy due to patchy retinal pigment epithelium atrophy was also present in all patients. The mean central retinal thickness observed in optical coherence tomography was 148 µm. Conclusions The identification of a third mutation in PRCD confirms its role in the pathogenesis of RP. Clinical findings were in line with the morphological changes observed in previous studies. Bull’s eye maculopathy seems to be a hallmark of RP due to mutations in the PRCD gene.

Visual field tests were performed using an Octopus 900 perimeter (Haag-Streit International, Köniz, Switzerland). Semiautomated kinetic perimetry using Goldmann stimulus V4e, III4e, and an additional dimmer stimulus, if possible, within the 90° visual field was performed in all patients.
All electrophysiological recordings were performed according to International Society for Clinical Electrophysiology of Vision Standards (ISCEV) [6,7]. Ganzfeld ERGs were performed using Dawson-Trick-Litzkow (DTL) fiber electrodes with an Espion E 2 or E 3 (Diagnosys LLC, Cambridge, UK) recording device coupled with a Color-Dome (Diagnosys LLC) as light source. Multifocal ERG was performed with an Espion System (Diagnosys LLC). The stimulus, consisting of 61 scaled hexagonal elements, covered a central visual field of 60° × 55°. Responses were amplified (200,000X), bandpass-filtered (10-100 Hz), and analyzed according to ring averages. The same DTL electrodes as those for the Ganzfeld recordings were used. Detailed morphological examination, color and infrared fundus photography, autofluorescence, and spectral domain optical coherence tomography (OCT; Heidelberg Engineering GmbH, Heidelberg, Germany) were performed in all patients.
Genetic analysis: Genomic DNA was extracted according to standard procedures. Peripheral blood was drawn in EDTA-blood tubes and stored at 4 °C for short-term and -20 °C for long-time storage. DNA was extracted with the automated chemagic MSM I system applying the chemagic DNA blood kit out of 5ml EDTA-blood (chemagen, Perkin Elmer, Baesweiler, Germany). Genetic testing for all genes associated with autosomal recessive retinitis pigmentosa was conducted in a diagnostic set-up (CeGaT GmbH, Tübingen, Germany) applying next-generation sequencing on a SOLiD 5500×l platform. Briefly, sequence reads were analyzed using LifeScope software package (Life Technologies GmbH, Darmstadt, Germany) and mapped to the human reference genome (GRCh37/hg19). Variant calling was generated by the LifeScope software package and annotated using the Ensembl database, Single Nucleotide Polymorphism database (dbSNP), and CeGaT in-house variant databases. Variants with a global minor allele frequency of below 5% based on the dbSNP and Exome Variant Server were further evaluated. All mutations were validated with conventional Sanger sequencing. Underrepresented regions (>10 reads per base) were examined with Sanger sequencing (detailed description of the genetic test is described in detail by Glöckle et al [8]). Segregation analysis in the three affected siblings and parents was performed with Sanger sequencing.

RESULTS
Clinical findings: In a family of five siblings (four women, one man), three were affected by RP (two women, one man). All patients noticed difficulty with night vision approximately at the age of 8 to 10 years. Their parents were first-degree cousins and had normal vision. There were no other cases of RP known in the family.
All three patients showed advanced signs of disease with reduced central visual acuity, color confusion, and severe constriction of the visual field (Table 1, Figure 2A-C). Nystagmus has never been reported, and pupillary reactions were normal in all cases. Electrophysiological examinations did not yield reproducible responses in any patient. Anterior segment examinations revealed subcapsular posterior cataract in each case. Typical findings on funduscopy were bone spicule-like pigmentation, attenuated arterioles, and optic disc pallor, all characteristic of RP ( Figure 2D-F). Interestingly, bull's eye maculopathy (BEM) due to patchy or geographic atrophy of the retinal pigment epithelium (RPE) was observed in every patient. This could nicely be seen in the autofluorescence images as sharply demarcated, smaller and larger, partly confluent areas of reduced autofluorescence ( Figure 2G-I). OCT imaging showed an overall decrease in retinal thickness with shortening of the photoreceptor outer segments, reduction in the outer nuclear layer, and RPE atrophy (Table 1, Figure 2J-L). The inner retinal layers were less affected.
Genetic findings: Diagnostic testing via next-generation sequencing and confirmation with Sanger sequencing in the index patient (IV:3) identified an apparent homozygous nonsense mutation c.52C>T p.R18X in exon 1 of the PRCD gene ( Figure 3A), as well as a heterozygous sequence variant c.1694T>C p.V565A in exon 1 of the interphotoreceptor retinoid-binding protein (RBP3) gene ( Figure 3B). Segregation analysis in the two affected siblings and parents via Sanger sequencing showed that both siblings were also homozygous for the PRCD mutation and heterozygous for the RBP3 variant. Both parents were heterozygous for the PRCD mutant allele.

DISCUSSION
PRCD is a canine form of autosomal recessive photoreceptor degeneration that serves as an animal model for RP in humans. The disease-relevant gene was mapped to the centromeric region of the canine chromosome 9, a region orthologous to the telomeric end of the human chromosome 17 [9,10]. Zangerl and coworkers showed that PRCD encodes for a small 54 amino acid protein in the dog and human, and a 53 amino acid protein in the mouse [3]. The first 24 amino acids are highly conserved in various vertebrate species, while PRCD could not be identified in non-vertebrate species. They further found an approximately equal expression of canine PRCD in the RPE, photoreceptor, and ganglion cell layers. Heterologous expression experiments showed that wild-type PRCD is distributed diffusely throughout the cytoplasm, while mutant protein aggregated. Zangerl and coworkers proposed that PRCD has a critical role in the maintenance of rod photoreceptor structure, function, and/or survival; however, the gene's function remains unknown [3].
To date, only two RP-causing mutations of the PRCD gene in humans have been reported. To date only two RP-causing mutations of the PRCD gene in humans have been reported. Zangerl et al. found a homozygous mutation in PRCD (c.5G>A p.C2Y) in a single human patient with autosomal recessive RP from Bangladesh [3]. The patient  showed typical functional and morphological characteristics of the disease, such as attenuated arterioles, extensive bone spicule-like pigmentation, and pale deposits at the level of the RPE. Additionally, small patches of geographic atrophy near fixation in both eyes were observed [3].
Nevet et al. [5] examined 24 patients from an Israeli Muslim Arab village affected by RP and found a second pathogenic homozygous nonsense mutation (c.64C>T p.R22X) in PRCD. The relatively frequent existence of an autosomal recessive disease in an isolated population suggested a founder effect. The ophthalmologic evaluation confirmed the diagnosis of RP in all affected individuals. In addition to the characteristic morphological changes, signs of macular degeneration including macular edema, macular pucker, scarring and BEM were observed in the majority of patients.
We found a novel homozygous nonsense mutation in exon 1 of the PRCD gene in three siblings affected by RP: c.52C>T, p.R18X. The mutation produces a premature termination codon at amino acid position 18 of the PRCD polypeptide, possibly resulting in a truncated and nonfunctional protein. However, more likely, there is no such polypeptide as the transcript is predicted to undergo nonsense mediated decay. A second unknown heterozygous sequence variant was also observed in all affected siblings in exon 1 of RBP3: c.1694T>C, p.V565A. This missense mutation alters an evolutionary highly conserved valine residue into an alanine, and prediction programs consider this variant pathogenic (Mutation Taster). However, no other sequence variant in RBP3 was identified, and consequently, the homozygous nonsense mutation in PRCD was considered causative for the autosomal recessive phenotype in this family, while the RBP3 variant was thought not to be responsible for the disease. Neither variant has been reported in the literature, and is not present in any database (i.e., Exome Variant Server, dbSNP). However, the RBP3 missense mutation could have a modifying effect on the RP phenotype.
Our patients presented with similar findings in the ophthalmological examinations as reported in previously described cases: myopia, posterior subcapsular cataract, bone spicule-like pigmentation, and attenuated arterioles were typical findings. All of our patients showed an advanced stage of disease. Interestingly, BEM due to patchy RPE atrophy was also present in all patients. The signs of macular degeneration were especially visible in the two female patients. This is in line with the morphological changes observed in the cases examined by Zangerl and Nevet [3,5]. BEM seems to be a hallmark of RP due to mutations in the PRCD gene.
BEM is a distinctive macular phenotype characterized by annular RPE atrophy with central sparing of the fovea. Initially, BEM was described in association with chloroquine retinopathy [11] but has been associated with various inherited retinal degenerations, also with RP [12][13][14]. However, in retinitis pigmentosa is not a common feature, and therefore, it is remarkable that nearly all patients with PRCD mutations reported in the literature carry this phenotypic characteristic.

ACKNOWLEDGMENTS
The diagnostic genetic testing was performed by CeGaT GmbH, Tübingen, Germany. We are grateful to the patients for their participation and thank the technicians Heike Enderle, Elke Feil, and Ulrike Fuchs for performing excellent examinations. The study was supported by a BMBF grant (01GM1108A) to SK. The authors have no commercial interests in the subject of the manuscript or in entities discussed in the manuscript.